Process of magnesium production



March 16, 1948. J, HANSGIRG 2,437,815

PROCESS OF MAGNESIUM PRODUCTION Filed Jan. 19, 1946 INVENTOR. 72/72 f/iQIA/J (1/24.

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atented Mar. 16, 1948 muse STATES PATENT OFFICE PROCESS OF MAGNESIUM PRODUCTION Fritz .l. Hansgirg, Black Mountain, N. 0., as-

signor to The Permanente Metals Corporation, Oakland, Calif., a corporation of, Delaware Application January 19, 1946, Serial No. 642,384

Claims.

This invention relates to the manufacture of magnesium. More particularly it relates to the manufacture of magnesium by reduction of magnesium oxide with carbon and is a continuationin-part of my copending application Serial No. 427,175, filed January 17, 1942, and entitled Process for magnesium production.

In order to effect reduction of magnesium oxide with carbon, it is necessary to achieve a temperature in excess of the boiling point of the metal. As a consequence the magnesium leaves the reduction zone in the vapor state along with an equal amount of carbon monoxide which is concurrently formed according to the following reaction:

The above reaction is reversible between the lower limit at which the reduction reaction takes place, approximately 1850 C., and the upper limit at which magnesium and carbon monoxide are stable in the presence of each other, approximately 450 C.

The ease by which back-reaction takes place delayed for many years developments in the production of magnesium by reduction of magnesium oxide with carbon. As a matter of fact it Was never possible to produce more than traces of magnesium by this process, the so-called carbothermic process, until the development of the process embodied in U. S. Patent 1,884,993 to Hansgirg. Prior to that time the art had unsuccessfully attempted to avoid reversion of the reaction by injecting indifierent gas into the reduction zone.

Hansgirg was successful in recovering good yields of magnesium according to the carbothermic process by shock-chilling with hydrogen gas the mixture of vapors and gases instantaneously as it left the reduction zone. By this process, which is embodied in the above patent, the magnesium and carbon monoxide are chilled to a point below 450 C. where they are stable in the presence of each other. However, the use of hy drogen as a chilling medium presents several disadvantages. Hydrogen is a rather expensive gas so that it is not economic to use it as a chilling medium unless it is used in recycle, and to recycle requires that carbon monoxide be removed. This latter operation is very difficult to eifect with hydrogen and as a result is quite costly. Furthermore, a hydrogen system is dangerous and expensive to construct and maintain because of the rate of diffusion of hydrogen. For example,

.it is necessary to have welded joints and seams throughout a hydrogen system. The likelihood of explosion is always present.

' Following the development of the hydrogen quench it was proposed to employ liquid hydrocarbons as the shock-chilling medium. There have been several developments along these lines. The principal advantage is claimed to lie in the feature that the condensed solids, which are inherently pyrophoric, are so wetted down with the liquids that they can be handled in the atmosphere without fear of combustion. However, this process also possesses disadvantages in that the pastelike material comprising the condensed products is difiicult to process for the purpose of extracting metallic magnesium therefrom. The magnesium is generally contaminated and the pasty residue left from extraction processes presents'an undesirable handling problem.

It has also been proposed to use solid materials as shock-chilling media such as lead, and magnesium chloride salts, or even cold surfaces. These developments have never attained commercial success because of poor chilling efficiency, the contamination of the recovered magnesium and the difficulty of effecting its recovery from the solid materials.

Recently the art has proposed the use of a dry hydrocarbon vapor or gas as a method of preventing undesirable back-reaction of magnesium vapors. This process is embodied in U. S. Patent No. 2,242,721, to Hanawalt et al. It differs from V the previous proposals in that it purports to react hydrocarbons, such as dry vapors of volatile hydrocarbons or even natural gas, with the magnesium vapors emerging from the furnace to thereby form magnesium carbides and some elemental magnesium with only traces of magnesium oxide.

The object is to form substantial amountsof carbide which are stated to render the condensate n-on-pyrophoric. Apparently no back-reaction occurs since it is asserted that only traces of mag- "nesium oxide are found in the furnace condensate. This process also has its disadvantages because in producing the carbides the condensate naturally has a poor rate of reactivity both as applied to chemical reactions and to rate of distilla tion when compared with the product of the present invention. As a consequence, it is more dimcult to sublime to recover pure metal because the limiting rate of recovery is the rate at which the carbides break down into free magnesium and carbon. The sublimation step yields a heavy residue of carbon which is diflicult to handle and represents a major cost element. No carbon dioxide can be tolerated in the chilling gas. The

capabilities of the precipitate as a highly reactive magnesium powder are reduced or lost when substantial amounts of the carbide are formed. The material is of practically no use as an incendiary. By the reaction in which the hydrocarbon is broken down to form carbides considerable amounts of free hydrogen are produced resulting in the disadvantages of the hydrogen system as discussed above.

Among the objects of the present invention is the production, by shock chilling with natural gas or like hydrocarbon gas, of a highly reactive furnace condensate which may be readily sublimed, or which may be used as an incendiary material, or as a raw material in the preparation of various chemicals such as magnesium nitrides and the like. Another object of the invention is to produce a dual method of shock chilling which prevents reversion of the reaction between magnesium and carbon monoxide and simultaneously passes through the range of carbide formation so rapidly as to avoid formation of amounts of carbides which would 'afiect the reactivity of the product from the reaction between mag n'esium and CH4 or natural gas and avoids break down of the natural gas to form magnesium carbides and hydrogen. Another object is to produce a precipitate material which is capable or being readily pelleted prior to sublimation or other treatment. Further objects are to avoid the formation of magnesium carbides in such amounts as would tend to decrease the activity of the condensate and also to avoid the forma tion of objectionable amounts of magnesium oxide. yield of magnesium forming mixture of mag nesium and carbon monoxide at a temperature of 1850 C. and above. Another object is to pro duce a pyrophorous product. These and other objects will be apparent from the description of the invention herein.

According to the present invention, dry natural gas is employed as the medium for sudden chilling (shock ch lling) of a mixture of magnesium vapor and carbon monoxide to a point where these components are stable in the presence of each other. The natural gas is injected into the mixture cf magnesium vapor and carbon monoxide gas as it passes from the temperature of the reduction zone, that is, at least approximately 1850 C., in a practically instantaneous manner, so that the materials reach in a fraction of a second a point where they are stable in the presence of each other, that is, at or below 450 0., preferably by reduction of the temperature to about 200 C.

A suitable arrangement of furnace outlet and gas injection mechanism for injecting the cold natural gas into the stream of gas and vapor issuing from the furnace is shown in my prior Patent No. 2,109,841. Alternatively, the nozzle and injection mechanism and method disclosed in the copend'ing application or Rhoades, Ser. No; 585,884, filed March 31, 1945, may be employed to obtain the desired shock chilling action in this connection.

Any natural gas of selected composition, free of undesirable constituents, such as nitrogen and carbon dioxide maybe employed. The relatively cool natural gas must be dry, and hence should be dried before use, since water reacts violently upon magnesium. Obviously, such steps as are required to free the particular natural gas of undesirable constituents may be employed.

The condensate produced by this process is a Another object is to produce a higher finely divided mixture of magnesium, magnesium oxide, and carbon with minor proportions of other materials. Except for carryover of unreacted materials, the particles of the mixture are of submicron size. When good chilling efficiency is obtained about 50 per cent. of the condensate is elemental magnesium with the remainder magnesium oxide and carbon, and other impurities. The MgO and C are present in the ratio of about equal or up to 3 parts of the former to 2 parts of the latter. The magnesium carbide content is negligible and at most does not exceed about 8 per cent. Negligible quantities of calcium oxide and sodium are also present.

The following data are presented to illustrate the operation of a carbothermic reduction furnace utilizing natural gas the shock-chilling agent during a typical days run.

The furnace feed is an intimate mixture of sea-Water magnesia and petroleum coke, which are ground together to a very fine powder and then pressed into hard, non-dusting pellets. The feed has the following analysis:

Per cent MgO A 69.2 Fixed Carbon 21.0 Impurities 5.5 Volatile 4.3

This feed is reacted in a three-phase electric furnace which is sealed from the atmosphere and operating at approximately 2000 C. Besides the feed, the materials fed to the furnace include metallurgical coke and graphite in the form of electrodes. Metallurgical coke is used to provide an electrically conductive bed in the furnace, sufiicient being added to maintain the level for proper operation. The electrodes are gradually consumed by the action in the furnace, and provision is made to feed them into the furnace as required. The electrodes and the coke represent a source of additional carbon as well as impurities in the reaction.

At the temperature of the furnace, the magnesia is reduced by the carbon to form magnes'ium metal (as a vapor) and carbon monoxide gas. However, between about 1800 C. and 450 C. the back-reaction of magnesium and carbon monoxide to form magnesia and carbon proceeds very rapidly, and consequently the gases coming away from the reaction zone must be practically instantaneously cooled below the temperature at which the back-reaction is dominant and rapid. In practice, it is found desirable to introduce suificient shock chilling gas at the available temperature that the instantaneous or shock chilling will reduce the mixed products to about 200 C. One reason for this is to ensure that the rate of chilling or temperature reduction of the mixed products as they approach the lower limit will not slow down in the temperature region where either back-reaction of magnesium vapor with carbon monoxide to form magnesium oxide or the interaction of magnesium and hydrocarbons to form magnesium carbide, might still occur. 'In other words, the curve of temperature reduction must fall steeply through the region of back-reaction and of carbide formation.

The cooling agent is natural gas, which has been treated to remove carbon dioxide and water vapor which would oxidize the extremely finely divided magnesium metal rapidly. Since relatively small amounts of carbon monoxide-below about 20 %-can be tolerated in the chilling gas provided the chilling is rapid enough and to a The product of the furnace reaction, after chilling, is a very fine dust, which has the following analysis:

Percent Mg 52.9 MgO 18.7 C 21.7 Impurities 6.8

The above analysis includes total magnesium available from free magnesium and magnesium carbide and similarly the carbon content shown includes that available from carbide. As previously mentioned, this carbide content does not exceed about 8 per cent.

The efficiency of the chilling in preventing the back-reaction may be indicated by a simple calculation;

Let Mg represent Mg. in the'dust expressed as equivalent MgO, or 1.66 Mg%.

Let MgO represent MgO in the dust.

Then: Efiiciency=100 Mg (Mg plus MgO).

Since this figure includes the efficiency of reduction in the furnace which is below 100 per cent. due to mechanical carryover of unreacted feed containing MgO, the true chilling efficiency will always be somewhat higher than the result obtained from the above calculation. present case, the indicated efiiciency is 82.5%.

During the 24-hour period the reduction furnace was in operation for 21 hours and 8 minutes. In this time 64,740 pounds of feed of the composition given above were treated at an average rate of 3070 pounds per hour and at an average furnace temperature of 1948 C. to produce approximately 41,610 pounds of carbothermic dust of the composition given. 130.000 kilowatt hours of electricity were used, or approximately two kilowatt hours per pound of feed. Carbon monoxide in the chilling gas average 8.1%, and the average flow of chilling gas to the chilling cone was 908.000 cubic feet per hour at a temperature of 118 F. This flow rate maintained the temperature of the furnace products in the chilling cone at an average of 424 F. An average of 218,000 cubic feet per hour of natural gas was added to the system, a corresponding quantity of the filtered chilling gas being rejected to be used as fuel. Thus, an average of 690,000 cubic feet per hour of the chilling gas was recycled, being cooled in heat interchangers before being used again.

When compared at standard temperature of 60 F. the volume of chilling gas used in the above run per unit volume of gaseous furnace product was in the ratio of approximately 20 to l, or when compared at the conditions at which they enter the chilling zone, the present process uses approximately 3 volumes of chilling gas per volume of gaseous furnace products.

As previously noted, the condensate is very pyrophoric. For example, it will spontaneously ignite when exposed to the atmosphere. It has In the thus'found wide use as an incendiary. The high activity of the dust makes it practically inextinguishable. Water only increases the rate of combustion and carbon dioxide has no deterrent effect. Difficulties in handling the dust are overcome by keeping it in an inert system, preferably a natural gas system, or even under certain conditions wetting it down with a liquid hydrocarbon.

The condensate can be converted into solid magnesium of high purity by any of the known distillation or sublimation processes. Satisfactory results have been achieved by using the process and apparatus disclosed in U. S. Patents 2,309,643, 2,309,644 and 2,310,188 to Hansgirg. Due to its high activity the dust may be employed as a raw material in many chemical processes, for example, in the preparation of magnesium nitrides and other magnesium compounds which have many industrial uses.

The physical character of the condensate, or dust, is illustrated by Fig. 1, which is an electron micrograph taken at a magnification of 32,000 diameters. It will be observed that a considerable proportion of the dust shows up as submicronlc spheres of pure magnesium metal relatively free of surface coating. A further portion of the magnesium is held in the form of agglomerates containing carbon, magnesium oxide, and other impurities. The diluent materials, i. e., carbon, magnesium oxide, and other impurities, including in this case magnesium carbides, are so finely divided that no structure can be observed even at this extremely high magnification.

It is apparent from the photograph that most of the metal is in a free state, and does not possess a surface coating. It is the presence of this free metal of such minute particle size that gives the dust its pyrophoric character and high chemical reactivity. Only slight evidence is shown for the beginning of crystal structure in the magnesium and this is only on the surface of the spheres. The retention of a true spherical structure and absence of crystal formation shows that the shock-chilling has been extremely eifective and the temperature of the dust mixture has passed from the dew point to a temperature below 650 C., the melting point of magnesium, so rapidly that freezing occurred before rearrangement to crystals could take place.

In the shock-chilling step of the invention the natural gas does not react with the magnesium vapors to form magnesium carbides except in negligible amounts. This is different from the prior art process, and is not obvious because it is Well known that magnesium vapor and hydrocarbons will react to form magnesium carbides. However. it is probably explained by the fact that the shock-chilling step is carried out in such manner that in traversing the range of instability for magnesium vapors and carbon monoxide gases, that is, from roughly 1850 C. to about 450 0., the region in which magnesium reacts with hydrocarbons to form carbides is also passed. This is possible only by such rapid mixing of cooling gas with vapors that chilling is substantially instantaneous. The process thus constitutes a double shock chilling, i. e., shock chilling to retard the reversible reaction of magnesium and carbon monoxide and to retard the reaction of methane or other hydrocarbons with magnesium metal.

The range of temperatures at which hydrocarbons react with magnesium to form carbides varies from 480 C. to 760 0., depending upon the hydrocarbon. In the case of methane, the temperature at which carbides are formed approaches the upper limit of this range. MgCa forms first and is practically completely converted into the other carbide MgzCs with increasing temperatures. Above 700 C. MgzCs begins to break down appreciably into Mg and C. These figures concerning carbide formation are the result of the work of J. Novak which was published in Zeitschrift fuer physikalisc'ne Chemie, 73, 1910, at pages 513 to 546 (ChemicalAbstracts, vol. 5, part 1, 1911). Accordingly, even though natural gas and magnesium vapor react to form carbides, in the process of the present invention the rapid cooling effect of the natural gas prevents the carbide reaction from occurring to any noticeable extent.

This concept of double shock chilling, i. e., rapidly passing through the critical ranges of both the reversion reaction and the carbide formation is entirely diiferent from and incompatible with the prior art concept of maximum carbide formation according to which the intention is to give full opportunity including ample time for the hydrocarbon to interact with the magnesium vapor.

The use of natural gas as a shock-chilling medium possesses several advantages over the use of hydrogen, which is a substantially inert quenching medium. The natural gas has a con siderably higher heat capacity per volume than hydrogen so that to accomplish a given cooling effect smaller quantities of natural gas are required. The natural gas has a lower diffusion rate than hydrogen, with the result that it is easier to maintain within a closed system than hydrogen. It is a considerably less hazardous gas to handle than hydrogen. Carbon monoxide is more readily separated from natural gas than it is from hydrogen, so that recycling of the chilling gas is considerably less diflicult with natural gas. Furthermore, the natural gas is of such low cost and is so plentiful that the mixture of natural gas and carbon monoxide issuing from the furnace after separation from the dust can be employed as a fuel for industrial purposes, such as conducting calcining operations in cement kilns and the like.

The use of natural gas in such a manner that it functions as a practically inert coolant for magnesium vapors instead of as a reactant is also quite an advantage. The high activity of the magnesium dust produced by the process of the present invention makes it considerably easier to refine by distillation and sublimation than a condensate containing large quantities of magnesium carbides because it is necessary to break down the carbides into elemental magnesium and carbon before pure magnesium is recovered. For the same reason the dust of this invention is also more desirable in certain chemical reactions, such as the formation of magnesium nitrides, a highly useful product as embodied in U. S. Patent applications Serial No. 577,586 of Byrns, filed February 12, 1945, and Serial No. 574,758 of Von Stroh, filed January 25, 1945. It is also to be noted that the condensate of this invention after sublimation yields a highly useful by-product comprising magnesium oxide and carbon having application as a compounding agent in the production of synthetic rubber as embodied in U. S. Patent application Serial No. 527,347 of Von Stroh, filed March 20, 1944, now abandoned, and Serial No. 565,423 of Byrns, filed November 27, 1944, now patent 8 2,410,267, dated October 29, 1946. The process of the prior art which employed natural gas does not yield such a product because its furnace condensate does not contain any carbon and-only traces of magnesium oxide.

What'is claimed is:

1. In the production of metallic magnesium comprising the reduction of magnesium oxide by carbon, wherein a mixture of magnesium vapor and carbon monoxide is produced and the magnesium vapor is cooled to condense solid magnesium powder, the step which comprises introducing relatively cool dry natural gas into the mixture of magnesium vapor and carbon monoxide gas emerging from the reduction zone and instantaneously shock-chilling the magnesium and carbon monoxide to a point where they are stable in the presence of each other and thereby simultaneously preventing the magnesium from reacting with the natural gas to form magnesium carbides and avoiding the formation of objectionable amounts of magnesium oxide by backreaction of magnesium and carbon monoxide.

2. In the production of metallic magnesium comprising the reduction of magnesium oxide by carbon, wherein a mixture of magnesium vapor and carbon monoxide is produced in a reduction zone maintained at at least about 1850 C., and the magnesium vapor is cooled to condense solid magnesium powder, the step which comprises introducing relatively cool dry natural gas into the mixture of magnesium vapor and carbon monoxide emerging from the reduction zone and instantaneously shock-chilling the magnesium and carbon monoxide to below about 450 C. where they are stable in the presence of each other, and thereby simultaneously preventing the magnesium from reacting with the natural gas to form magnesium carbides and avoiding the formation of objectionable amounts of magnesium oxide by back-reaction of magnesium and carbon monoxide.

3. In the production of metallic magnesium by carbothermic reduction, the step of shock chilling which consists in injecting relatively cool dry natural gas into a mixture of magnesium vapor and carbon monoxide to bring the temperature of the said mixture substantially instantaneously from substantially 1850 0. down to substantially 200 C., whereby to pass with great rapidity through the range of back-reaction temperatures of magnesium vapor and carbon monoxide, i. e., about 1850 C. to 450 C., and through the range of interaction temperatures of magnesium with the hydrocarbon constituents of the natural gas to form magnesium carbide, i. e., about 760 C. to 480 C. with the precipitation of fine dust consisting of approximately 50% by weight metallic magnesium and not exceeding about 8% by weight magnesium carbide.

4. In the production of metallic magnesium by a process comprising the evolution of a mixture of magnesium vapor and carbon monoxide by the reduction of magnesium oxide by carbon and the condensation of the magnesium vapor to solid magnesium powder; the step which comprises suddenly injecting into the mixture of magnesium vapor and carbon monoxide, at the point of its discharge from the reduction zone, a sufiicient quantity of cool dry natural gas in fine jets to instantaneously chill the mixture of magnesium vapor and carbon monoxide to a temperature of about 200 C. to obtain metallic magnesium which is stable in the presence of carbon monoxide and natural gas and thereby pass through the intermediate temperature range of about 700 C. to 480 C. before any appreciable amount of magnesium carbide formation can occur and minimize the tendency of magnesium and carbon monoxide to back-react to form ma nesium oxide and carbon.

5. In the production of metallic magnesium by a process comprising the evolution of a mixture of magnesium vapor and carbon monoxide by the reduction of magnesium oxide by carbon and the condensation of the magnesium vapor to solid magnesium powder; the step which comprises suddenly injecting into the mixture of magnesium vapor and carbon monoxide, at the point of its discharge from the reduction zone, a quantity of cool dry natural gas in such proportion that approximately 20 volumes of natural gas are used per volume of the hot mixture of magnesium vapor and carbon monoxide gas measured at normal pressure and 60 F., to instantaneously chill the mixture of magnesium vapor and carbon monoxide to a temperature of about 200 C. to obtain metallic magnesium which is stable in the presence of carbon monoxide and natural gas and thereby pass through the intermediate temperature range of about 700 C. to 480 C. before any appreciable amount of magnesium carbide formation can occur and minimize the tendency of magnesium and carbon monoxide to back-react to form magnesium oxide and carbon.

6. A process for producing substantial amounts of metallic magnesium comprising reducing magnesium oxide by carbon in a reaction mixture at a temperature of at least about 1850 C. to form a mixture of magnesium vapor and carbon monoxide gas, instantaneously chilling said mixture to a temperature at which magnesium and carbon monoxide are stable in the presence of each other by injecting large quantities of cool dry natural gas into said mixture at the point of its emergence from the reduction zone, separating the chilled magnesium from the carbon monoxide, and recovering a product comprising substantial amounts of metallic magnesium in admixture with substantial amounts of back-reacted and unreacted magnesium oxide, carbon and insignificant amounts of other impurities.

'Z. A process for producing substantial amounts of metallic magnesium comprising reducing magnesium oxide by carbon in a reaction mixture at a temperature of at least about 1850 C. to form a mixture of magnesium vapor and carbon monoxide gas, instantaneously chilling said mixture to a temperature at which magnesium is condensed to a fine powder and is stable in the presence of carbon monoxide by injecting large quantities of cool dry natural gas into said mixture at the point of its emergence from the reduction zone, separating the chilled magnesium powder from the carbon monoxide, and recovering a product comprising substantially entirely metallic magnesium, back-reacted and unreacted magnesium oxide and carbon, with only insignificant amounts of other impurities, wherein the amount of metallic magnesium is approximately equal to the amount of magnesium oxide and carbon.

8. A process for producing substantial amounts of metallic magnesium comprising reducing magnesium oxide by carbon in a reaction mixture at a temperature of at least about 1850 C. to form a mixture of magnesium vapor and carbon monoxide gas, instantaneously chilling said mixture to a temperature at which magnesium is condensed to a fine powder and is stable in the presence of carbon monoxide by injecting large quantities of cool dry natural gas into said mixture at the point of its emergence from the reduction zone, separating the chilled magnesium powder from the carbon monoxide, and recovering a product wherein the amount of metallic magnesium is in the range of about 50 percent by weight of the product, and theremainder, with the exception of small amounts of other impurities, consists of magnesium oxide and carbon in the weight ratio of about 3 to 2.

9. A process for producing substantial amounts of metallic magnesium comprising reducing magnesium oxide by carbon in a reaction mixture at a temperature of at least about 1850 C. to form a mixture of magnesium vapor and carbon monoxide gas, instantaneously chilling said mixture to a temperature at which magnesium is condensed to a fine powder and is stable in the presence of carbon monoxide by injecting large quantitles of cool dry natural gas into said mixture at the point of its emergence from the reduction zone, separating the chilled magnesium powder from the carbon monoxide, and recovering a product comprising about 50 percent metallic magnesium, about 40 percent of total magnesium oxide and carbon, and about 10 percent of other impurities.

10. A process for producing substantial amounts of metallic magnesium comprising reducing magnesium oxide by carbon in a reaction mixture at a temperature of at least about 1850 0. to form a mixture of magnesium vapor and carbon monoxide gas; instantaneously chilling said vapor mixture to a temperature of about 200 0., at which magnesium is condensed to a fine powder and is stable in the presence of carbon monoxide, by injecting about 20 volumes of cool dry natural gas for each volume of said vapor mixture measured at normal pressure and temperature, said natural gas being injected in the form of fine streams at high pressure at the point of its emergence from the reduction zone; separating the chilled magnesium powder from the carbon monoxide; and recovering a product comprising about 50 percent metallic magnesium, about 40 percent of total magnesium oxide and carbon and about 10 percent of other impurities.

FRITZ J. HANSGIRG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,884,993 Hansgirg Oct. 25, 1932 2,240,817 Hansgirg May 6, 1941 2,242,721 Hanawalt et al. May 20, 1941 Certificate of Correction Patent No. 2,437,815. March 16, 1948.

FRITZ J. HANSGIRG It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Column 7, line 74, strike out the words and comma now abandoned, and insert the same in column 1, line 6, after January 17, 1942, and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 15th day of June, A. D. 1948.

THOMAS F. MURPHY,

Assistant flonwnissz'oner of Patents. 

